1. Field of the Invention
The present invention relates to the field of digital to analog converters.
2. Prior Art
Current steering digital to analog converters (DACs) are well known in the prior art. In a current steering DAC, current sources are provided, with each current source being coupled to an output load, or to a power supply terminal or dummy load, dependent on the bit controlling the current steering switch controlling the destination of the respective current source. In a binary coded current steering DAC, the current sources are in a binary progression, each in proportion to the respective bit in the binary coded input, so that appropriate summing of the currents of the “active” bits provides an analog current output proportional to the value of the binary coded input. Passing the output current through a resistor provides an analog voltage proportional to the value of the binary coded input.
In a thermometer coded current steering DAC, assuming a binary coded input, the binary code is first converted to a thermometer code, so-called because of the nature of a thermometer code. In particular, an N bit binary code recodes to a 2N thermometer code, with all bits up to particular level, dependent on the binary code input, being ones, or active so to speak, and all bits above that being zeros, or inactive. Each bit of the thermometer code is then used to control the steering of a respective one of 2N equal current sources to the output, or to a power supply terminal or dummy load. Again, passing the output current through a resistor provides an analog voltage proportional to the value of the thermometer code, or in turn, proportional to the value of the binary coded input.
In a segmented DAC with an N bit binary code input, the input is broken up into two or more segments, such as, by way of example, two approximately equal size segments for the most significant bits and the least significant bits, each segment being converted to analog form, with the outputs being combined for an overall DAC output. Usually a segmented DAC uses thermometer coding for at least the most significant bits.
A current steering DAC may have a single ended or differential output. In the case of a differential output, the various current sources are steered to a positive output if the respective bit is active, or to a negative output if the respective bit is not active.